This invention relates to a solid polyelectrolyte polymer film comprised of a single polymer sheet or film which contains electrochemical species and which can be used as a static dissipative film, or be connected to current collectors on each of its two sides, forming a secondary battery.
Most simple salts are not good electrical conductors. However, a variety of solids have high ionic conductivities which range from hard, refractory materials, such as sodium beta-alumina, through softer materials, such as silver iodide (AgI), to the very soft polymer electrolytes. Stoichiometric (AgI), nonstoichiometric (sodium .beta.-alumina), or doped compounds (calcia-stabilized zirconia) are included in the list. Most electrically conductive metals have values above 10.sup.5 ; whereas, insulating materials have typical values lower than 10.sup.-16 (ohm-cm).sup.-1.
At temperatures above 150 degrees C., silver iodide shows increased electrical conductivity which results from movement of silver cations. This can be shown by placing silver electrodes on either side of a silver iodide pellet. After the desired amount of time, the silver plates were reweighed. The negative plate gained silver while the positive plate lost silver, and that difference was the same as expected if silver cations carried the charge. It has been reported that even ionic solids such as sodium chloride have some electrical conductivity that are not electronic but ionic.
A model of how crystal lattice defects can promote ionic motion (diffusion) over long molecular distances has been proposed by Shriver et al., Solid State Electronics, 5, '83 (1982). If ionic solids had perfect ordered structure, every lattice site would be occupied by the appropriate ion. However, every structure is not perfect. Defects arise when ions exist in interstitial sites or when vacancies exist in sites which are normally filled. Frenkel disorder is defined as the hopping of ions through a series of interstitial sites, and Schottky disorder is defined as the hopping of vacancies through normal lattice positions. These disorders produce ionic conductivity in solids.
Many studies have shown that most crystalline inorganic electrolytes exhibit this behavior. All have a low-conductivity phase in which the ions are ordered. They exist in subsets of lattice sites. At higher temperatures these ions become disordered and ionic conductivity is increased. High conductivity depends on how fast ionic migration (transport) occurs. If the energy needed to disrupt ions in the available sites and needed to transport ions to vacant sites in the disordered phase is low, high ionic conductivities will result.
Compounds which contain a subset of ions which are located among a large group of vacant sites can undergo fast ionic transport if there is enough energy to disorder the ions among the available sites and if the energy is great enough to move ions from filled to vacant sites. If these energies are low, then, the conductivity can be high. Such a compound is betaalumina. For example, the sodium/sulfur battery operates utilizing this principle at 300 degrees C. A sodium beta-alumina ceramic tube is used to separate the molten sodium positive electrode from the molten sulfur negative electrode. Sodium is oxidized during discharge at the sodium/sodium beta-alumina interface. The resultant sodium cation is transported through the beta-alumina (solid electrolyte) where it eventually combines with reduced sulfur in the outer chamber to form sodium polysulfide. The sulfur electrode contains enough carbon to make it electronically conducting. During the charging mode, this process if reversed.
In all electrochemical cells, whether they are batteries or sensors, the electrolyte is essential since the principle electrochemistry occurs at the interface between the electrodes and the electrolyte. At the interface, a metal atom (electrode) can be oxidized to a metal cation which can enter the electrolyte while metal cations from the electrolyte can be incorporated into the other electrode such as titanium disulfide. Therefore, the electrolyte's role is to provide a path for the migration or diffusion of ions from one electrode to the other. This flow of charge can be offset or balanced by the flow of electrons through an external circuit. For example, a battery or a sensor requires that the electrolyte cannot conduct electrons or be electronic conductive. If it does conduct electrons, the battery would discharge (short-out) as it stands. In other words, the electrolyte must be a good ionic conductor and a very poor electronic conductor.
The primary reason why solid-state batteries have not been very successful is the dimensional changes which take place in the electrodes during charging and discharging. For example, a lithium negative electrode oxidizes as the battery discharges which slowly strips lithium metal away from the metal-electrolyte interface. If the interface cannot deform to maintain good interfacial contact, the battery will fail. Also, the insertion of lithium cations into the titanium disulfide positive electrode can swell the electrode which also diminishes interfacial contact. Instead of using hard, crystalline solid electrolytes which loose interfacial contact during discharge, soft, flexible polymeric electrolytes could deform or flow and continually maintain interfacial contact with the electrodes. Polymeric electrolytes can also be cast as thin films which can lower the resistance of the electrolyte, its volume and weight.
In an attempt to solve this problem and reduce the dimensions of secondary batteries, recent battery research has turned to the use of polymeric films in secondary batteries. See European patent application No. 84107618.5 June 30, 1984. The use of polymeric films can provide batteries having very thin crosssections and decreased weight.
In a further improvement, Noding et al. U.S. Pat. No. 4,714,665, teaching a three-layer polymeric film secondary battery, and U.S. Pat. No. 4,728,588, teaching a single polymer film layer in a secondary battery, provide additional diminution of the dimensions of the film layers. It has now been recognized that the polymeric film containing the electrolyte species used in the inventions of the foregoing patents is a separate and heretofore unclaimed invention.
It is therefore an object of this invention to provide a novel solid polyelectrolyte polymer film, which is useful in a secondary battery which incorporates the utilization of a single polymeric film and which, as a result, has a very thin cross-section even when constructed of a plurality of cells.